The sweeps were conducted in 16 cities nationwide over the past five days, authorities said.

"Our top priority in these cases has always been to identify children victims and move swiftly to remove them from these dangerous environments," FBI Director Robert Mueller said.

Mueller said this week's sweeps bring to 433 the number of child victims recovered in the five years since the FBI began its Innocence Lost initiative. The program was designed to combat a growing problem of underage prostitution.

"These kids are victims. They lack the ability to walk away. This is the 21st-century slavery," said Ernie Allen, president of the National Center for Missing and Exploited Children.

A University of Pennsylvania study found that an estimated 300,000 children in the United States are at risk of being sexually exploited for commercial uses, according to The Associated Press. Most of those children are "runaways or thrown-aways," Allen said.

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Allen said that about 1.6 million children run away from home each year, although most quickly return unharmed. Boys as young as 11 and girls as young as 12 are often targeted by prostitution rings

He praised the FBI and Justice Department for increased attention to fighting the problem, which he said is vastly underreported.

Allen said that until federal authorities began to crack down on use of child prostitutes in recent years, many pimps and prostitute rings who felt pressure from local police simply moved their operations to other jurisdictions. Watch Mueller say that hundreds of kids have been rescued

Authorities said that 290 of the 345 arrests made this week were adult prostitutes. Most will be charged with state crimes. However, increasingly, federal prosecutors have brought charges in cases involving juveniles because federal penalties are much more harsh than state penalties, officials said.

Justice Department and FBI officials noted two cases in which individuals trafficking in child prostitution received life sentences.